Problem: Twelve students in Mrs. Stephenson's class have brown eyes. Twenty students in the class have a lunch box. Of Mrs. Stephenson's 30 students, what is the least possible number of students who have brown eyes and a lunch box?
Explanation: Since $20$ students have a lunch box, $10$ students do not have a lunch box.  So at most $10$ students with brown eyes do not have a lunch box, and at least $\boxed{2}$ students with brown eyes must have a lunch box.